The long range objective of this proposal is to determine the factors which regulate cell differentiation so that we can have a better understanding of embryonic development and growth. To achieve this objective, we will perform studies on embryonic skeletal muscle cells. Our specific aims are to define the role of cell surface in regulating cell interactions and cell differentiation. Our apporach includes cell culture, monospecific antibodies to myogenic cells and immunobiochemistry. Monoclonal antibodies specific to embryonic myogenic cell surfaces have been isolated and quantitive changes in the determinants with which they react have been demonstrated during muscle cell development in vitro. We propose to use these antibodies to determine if distinct lineages of myoblasts give rise during development to muscles of different functional type; if specific determinants are required for cell recognition and fusion; and to determine the biochemical and immunological character of the surface alterations which distinguish myoblasts from myotubes and other embryonic cell types.